Designers of wireless communication systems presently use radio frequency (RF) models as tools to aid in their designing effort. An RF model is basically a mathematical simulation or characterization of some aspect or function of an RF communication system. Once formed, an RF model can be used to test the operation of the system that it models before having to incur the expense of building the system itself.
RF models have been used to test the behavior of RF signals traveling in an RF environment in which a communications system provides service. Such tests are designed to help determine whether scattering in the RF environment may cause the RF signal to degrade to the point that the RF signal can not be detected and/or decoded by receivers operating in the communications system. The term scattering refers to the phenomenon wherein an RF signal, traveling in an RF environment, hits and reflects off structures in the RF environment, thereby causing the RF signal to take random paths through the environment. The propensity of an RF environment to scatter signals is hereinafter referred to as "scattering hostility." Depending on the scattering hostility in an RF environment, an RF signal may multipath (i.e. simultaneously travel different paths between two points) as it travels in the RF environment.
By causing a signal to multipath, scattering can be problematic to maintaining reliable communications in a wireless system. To illustrate, an RF signal transmitted from a wireless terminal, such as a cell phone, actually propagates in all directions from the wireless terminal's antenna. A signal propagating in a first direction from the antenna may reflect off the side of a building and thereafter travel in a new direction in the RF environment. The same signal propagating in a second direction from the antenna of the wireless terminal may, however, travel in the RF environment without being reflected. When this happens, the non-reflected and the reflected signals may ultimately travel from the transmitting antenna to the same receiving antenna, but through different paths in the RF environment. As a result, they will typically be received by the receiver at different times. Thus, an incoming signal (to the receiver) can actually be composed of at least two so-called multipath components of the transmitted signal.
Depending on the characteristics of the multipath components of an incoming signal, a wireless communications system may have great difficulty performing certain functions. For example, the characteristics of the multipath components may cause some conventional geolocation systems to incorrectly determine the geolocation of a wireless terminal communicating with the system. The term geolocation as used herein refers to the point in two- or three-dimensional space defined by a set of coordinates, e.g. longitude and latitude, and/or defined by a vector, i.e. distance and direction, from a known point in space. To determine the geolocation of the wireless terminal, such geolocation systems must determine the time-of-arrival of the line-of-sight component of a signal transmitted by the wireless terminal.
To determine the time-of-arrival of the line-of-sight component of the incoming signal, such conventional geolocation systems assume that the first-arriving component of the incoming signal is the line-of-sight component. If, however, the time-of-arrival of the first-arriving component is very close in time to the time of arrival of the next arriving multipath component, the geolocation system may not be able to distinguish between the two components, thereby causing the geolocation system to mistakenly determine that the time of arrival of the line-of-sight component is at some time between the time of arrival of the first-arriving component and the time-of-arrival of the next-arriving multipath component. When this happens, the geolocation system will incorrectly assume that the line-of-sight component arrived at a later time than it actually arrived. This so-called time-shift will thereby cause the geolocation system to incorrectly base the calculation of the geolocation of the wireless terminal on a time-of-arrival that is "time-shifted," and thus cause the geolocation to make an incorrect geolocation calculation.
In addition, if scattering prevents the actual line-of-sight component from reaching the receiving unit (e.g. a building blocks the line-of-sight path), then the first-arriving component of the received signal will not be the actual line-of-sight component but, rather, some later-arriving multipath component of the transmitted signal. When this happens, the geolocation system may incorrectly assess the time-of-arrival of the line-of-sight component as being received at a later point in time, i.e. time-shifted, thereby causing the geolocation system to inaccurately calculate the geolocation of the wireless terminal.
One device that uses conventional RF models to identify or test the effects of scattering in an RF environment is a ray-tracing tool. In general, a ray-tracing tool uses an RF model to predict the path or paths that an RF signal would travel in an RF environment. To make such a prediction, the ray-tracing tool must be programmed with a description of the RF environment. Once programmed, the ray-tracing tool generates an RF model of the RF environment and uses that RF model to make a trace of all the paths that the RF signal would travel if the signal had actually been transmitted from a particular location in the RF environment. That is, the ray-tracing tool generates a ray-trace by simulating the propagation of the RF signal using the RF model of the RF environment.
Since the desired output of the ray-tracing tool is a ray-trace that includes every path that the given RF signal would travel in the RF environment, obtaining such a desired output means that the ray-tracing tool must be able to accurately identify every point of reflection, or scattering, that exists in the RF environment. This requires that the RF model be programmed with an accurate description of the physical composition of the RF environment. The required physical description, or so-called environmental information, includes the location of buildings and/or other structures (e.g. trees, towers, etc.), the physical size of the structures, the composition of the structures (e.g. the types of materials), the density of structures, and the physical contour of the terrain (e.g. hills, valleys, etc.).
For any given RF environment, however, such environmental information can be voluminous and difficult to obtain. Thus, a programmer can spend many hours gathering environmental information and programming the RF model with the environmental information, while never being assured that the RF model is detailed enough to enable the ray-tracing tool to output an accurate ray-trace. In addition, given the constant change of many urban and suburban environments (e.g. the construction and demolition of buildings, the erection of communications towers and/or antennas, and the leveling of highly wooded areas), a fully-programmed conventional RF model is only useful for a limited time.